Cylinder piston drive

ABSTRACT

The present invention is directed to a cylinder-piston drive, in particular an hydraulically controlled actuator ( 1 ) for actuating a gas-exchange valve ( 2 ) of an internal combustion engine, which has an operating piston ( 8 ) that is displaceable inside a cylinder ( 6 ) and delimits pressure chambers ( 10, 12 ) by piston sides ( 14, 16 ) facing away from one another, the operating piston ( 8 ) having a multipart design and being made up of at least two partial pistons ( 18, 20 ) that are placed inside one another, are displaceable relative to each other and strike against one another at stop faces ( 36, 38 ). One pressure chamber ( 10 ) is delimited by all ( 18, 20 ) and the other pressure chamber ( 12 ) is delimited by only a part of the partial pistons ( 20 ) and the sliding paths (s 1 ) of the partial pistons ( 18 ) not delimiting the other pressure chamber ( 12 ) is reduced compared to the overall sliding path (s 1 +S 2 ) of the operating piston ( 8 ), and at least one stop face ( 36 ), arranged on the cylinder ( 6 ), is provided against which a stop face ( 38 ) of one of the partial pistons ( 18 ) strikes after traveling the reduced sliding path (s 1 ).  
     The present invention provides that at least some of the stop faces associated with each other are designed as conical surfaces ( 36, 38 ) that in each case form a conical seat when struck. In this way, the leakage volume flow is reduced by the operating piston.

BACKGROUND INFORMATION

[0001] The present invention is based on a cylinder-piston drive, in particular an hydraulically controlled actuator for actuating a gas-exchange valve of an internal combustion engine, having an operating piston, which is displaceable inside a cylinder and which delimits pressure chambers by way of piston sides facing away from one another. The operating piston is made up of a plurality of parts and consists of at least two partial pistons, which are placed inside one another, are displaceable relative to one another and are able to strike one another at stop faces. One pressure chamber is delimited by all partial pistons and the other pressure chamber is delimited by only a part of the partial pistons. The sliding paths of the partial pistons not delimiting the other pressure chamber are reduced with respect to the overall sliding path of the operating piston, and at least one stop face, arranged on a cylinder, is provided, which a stop face of one of the partial pistons strikes after traveling the reduced sliding path, according to the definition of the species in claim 1.

[0002] Such a cylinder-piston drive is described in the heretofore unpublished German patent application 101 43 959.8 and relates to an hydraulically controlled actuator for actuating a gas-exchange valve. The actuator allows the effective areas of the operating piston, which open and/or close the gas-exchange valve, to be modified as a function of its sliding path, so that the actuating force acting on the gas-exchange valve may meet special demands, such as an initially high opening force of the actuator, so that the gas-exchange valve is able to open against the residual gas pressure, or a reduced closing force shortly before the valve closes, for noise and wear reasons.

SUMMARY OF THE INVENTION

[0003] According to the present invention, the stop faces are designed as conical surfaces forming a conical seat in each case, which has the result that the pressure chambers, which are separated from one another by the partial pistons guided inside each other, are sealed much more effectively. Therefore, the leakage volume flow that cannot always be entirely avoided in the case of multipart operating pistons is substantially reduced or completely eliminated. With respect to its leakage behavior, the multipart operating piston configured according to the present invention then no longer has any disadvantages compared to a one-piece operating piston. Given the same leakage volume flow as in a multipart operating piston that is not designed according to the present invention, it is possible, as an alternative, to allow larger manufacturing tolerances, in this way achieving lower manufacturing costs of the cylinder-piston drive. Since, in the case of conical seats, the associated conical surfaces are pressed together more and more as the pressure difference increases in the two pressure chambers, the sealing effect is advantageously self-enhancing.

[0004] Advantageous further refinements and improvements of the invention indicated in claim 1 are rendered possible by the measures specified in the dependent claims.

[0005] It is especially preferred if the cone angles of the associated conical surfaces have a slight angular difference and contact each other essentially in the form of a line contact. Such a conical seat, in which a line contact results because of a differential angle, is distinguished by an especially high tightness, since the line contact has the effect of a sealing edge being pressed, under prestress, against a sealing surface.

BRIEF DESCRIPTION OF THE DRAWING

[0006] An exemplary embodiment of the present invention is shown in the drawing and explained in greater detail in the following description.

[0007] The figures in the drawing show:

[0008]FIG. 1 A partial cross section through a preferred specific embodiment of a cylinder-piston drive according to the present invention in the form of an actuator for actuating a gas-exchange valve, in a valve-closed position;

[0009]FIG. 2 The actuator from FIG. 1 in a valve-open position.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0010] According to a preferred specific embodiment of the cylinder-piston drive according to the present invention, FIG. 1 shows a schematic part-sectional view of an hydraulically controlled actuator 1 for actuating a gas-exchange valve 2 of an internal combustion engine; it is shown in a position of normal use, i.e., the components shown at the bottom are also installed at the bottom. Gas-exchange valve 2 may be used as an intake valve for controlling an intake-cross section, and as a discharge valve for controlling a discharge-cross section. Gas-exchange valve 2 has a valve tappet 4 at whose lower end a valve disk is arranged (not shown here for reasons of scale), which cooperates with a valve-seat surface formed in a cylinder head of the internal combustion engine in order to lift it off, to a greater or lesser degree, from the valve-seat surface and to release a certain flow cross section via a linear actuation of valve tappet 4.

[0011] Hydraulically controlled actuator 1 has an operating piston 8, which is held in cylinder 6 so as to be axially displaceable and which acts on valve tappet 4. By end faces facing away from one another, operating piston 8 divides cylinder 6 into two hydraulic pressure chambers, namely into an upper pressure chamber 10 and a lower pressure chamber 12. The two pressure chambers 10, 12 are filled with hydraulic oil and are connected to a pressure-supply device via pressure lines. The end faces of operating piston 8 constitute effective areas for the hydraulic pressure present in pressure chambers 10, 12. Pressure chamber 12 is preferably always pressurized and pressure chamber 10 is preferably acted on by the same pressure in order to open gas-exchange valve 2 via the larger end face of operating piston 8 facing this pressure chamber 10, or to close it by reducing the pressure in pressure chamber 10. The fundamental functioning method of such an hydraulically controlled actuator 1 is known from DE 198 26 047 A1, for example, and will therefore not be discussed further here.

[0012] In contrast to the cited publication, operating piston 8 is designed such that there is a change in the surface area of the two effective areas along the sliding path of operating piston 8, so as to satisfy particular demands made on actuator 1 during opening and closing of gas-exchange valve 2. Such demands may be, for example, a high opening force at the beginning of the opening stroke of gas-exchange valve 2 in order to enable gas-exchange valve 2 to open against the residual gas pressure, and, on the other side, a marked reduction in the actuating force generated by actuator 1 following this fraction of the overall lift, so that the energy consumption required for controlling gas-exchange valve 2 is reduced.

[0013] In the case at hand, these demands are met in that operating piston 8 is designed in such a way that, in response to a displacement out of its valve-closed position shown in FIG. 1, upper effective opening area 14 is larger in the leading area s₁ of the displacement path than it is in the remaining sliding path S₂. To this end, upper effective opening area 14 gets smaller by a predefined amount following the specified sliding path s₁ and remains constant until completion of the lift. In contrast, lower effective closing area 16 of operating piston 8 remains generally constant across the entire closing lift s₁+S₂. Thus, gas-exchange valve 2 is opened with great displacement force, which then drops rapidly and remains constant over the remaining lift S₂.

[0014] To this end, operating piston 8 has a multipart design and is made up of a plurality of partial pistons, preferably two partial pistons inserted inside one another and displaceable relative to each other, namely an outer cylindrical piston 18 and an inner stepped piston 20. Stepped piston 20 is either integrally formed with valve tappet 4 or, as shown in FIG. 1 and FIG. 2, configured as an annular body, which has a stepped bore and is pressed onto likewise stepped valve tappet 4. Cylinder 6 also has a bored step 22, an upper cylinder section 24, which has a larger diameter, accommodating both partial pistons 18, 20, and a lower cylinder section 26, having a smaller diameter, guiding only stepped piston 20. Furthermore, cylindrical piston 18 has a smaller axial length than stepped piston 20 whose end faces face both upper pressure chamber 10 and lower pressure chamber 12, whereas only one end face of cylindrical piston 18, namely the upper end face, cooperates with a pressure chamber 10.

[0015] At its radially outer peripheral area, shorter cylindrical piston 18 is guided by upper cylinder section 24 and at its radially inner peripheral area by a cylindrical guide section 28 formed on a stepped piston 20, while stepped piston 20 is guided by lower cylinder section 26 of cylinder 6. The upper end, facing upper pressure chamber 10 and adjoining guide section 28, of stepped piston 20 has a reduced diameter so as to provide a radially outer stop face 30 for an associated radially inner stop face 32 of cylindrical piston 18, which is formed at an annular projection 34, as shown in FIG. 2.

[0016] By a radially inner stop face 36 formed at bore step 22 of cylinder 6, the sliding path of cylindrical piston 18 is limited in that it is provided with an associated radially outer stop face 38 at its end facing lower pressure chamber 12 (FIG. 1). In contrast, the sliding path of longer stepped piston 20 is able to traverse the overall lift s₁+S₂ of operating piston 8. Furthermore, bore step 22 of cylinder 6 completely decouples cylindrical piston 18 from lower pressure chamber 12. Space 39 between bore step 22 of cylinder 6 and cylindrical piston 18 is relieved to the point of ambient pressure.

[0017] When operating piston 8 is displaced out of its valve-open position shown in FIG. 1, in the valve-opening direction, this being effected by fluid pressure being input into upper pressure chamber 10, both partial pistons 18, 20 are first acted on by pressure and both are displaced downward. The opening upper effective area 14 of operating piston 8 is then made up of the two annular end faces of both partial pistons 18, 20 and is maximal. Once operating piston 8 has completed valve travel s₁, radially outer stop face 38 of cylindrical piston 18 strikes against associated stop face 36 of cylinder 6, so that cylindrical piston 18 no longer participates in the further displacement of operating piston 8. The effective opening area 14 is thus reduced to the end face of inner stepped piston 20 acted on by the fluid pressure, so that the displacement force of actuator 1 is reduced and the energy consumption drops during the further opening of gas-exchange valve 2.

[0018] If, upon reaching the open position of gas-exchange valve 2, the closing procedure is initiated by relieving upper pressure chamber 10, inner stepped piston 20 having traveled sliding path S₂, outer cylindrical piston 18 is carried along across sliding path s₁ by inner stepped piston 20, up to the closed position of operating piston 8, in that the two associated stop faces 30, 32 at stepped piston 20 and at cylindrical piston 18 come to rest against each other, as shown in FIG. 1.

[0019] As can be gathered from FIG. 1 and FIG. 2, respective associated stop faces 30, 32 and 36, 38 are designed as conical surfaces that, when striking against one another, form a conical seat 40, 42, the conical surfaces being pressed together or disengaging depending on the direction of the actuating force being exerted in each case. Specifically, according to FIG. 1 (valve-closed position), radially inner conical surface 32 of cylindrical piston 18 and radially outer conical surface 30 of stepped piston 20 form a conical seat 40 when striking against one another and, according to FIG. 2 (valve-open position), radially outer conical surface 38 of cylindrical piston 18 and radially inner conical surface 36 of cylinder 6 form an additional conical seat 42.

[0020] Associated conical surfaces 30, 32 and 36, 38 preferably have slightly different cone angles, so that they contact each other essentially in the form of a line contact, which, in the present case, has the form of a peripheral circular ring 44, 46. The cone angle difference between the associated conical surfaces 30, 32 and 36, 38 is shown in a highly exaggerated illustration in FIG. 1 and FIG. 2 for better visualization.

[0021] In a further development of described operating piston 8, it may also be constructed from more than only two partial pistons 18, 20. The individual partial pistons then have different lengths again and lose their effectiveness in the further movement of the operating piston by an appropriate definition of their valve travel, so that the effective opening area of the operating piston changes several times in the course of its overall valve travel. It is understood that the stop faces provided at the plurality of partial pistons are likewise designed as conical surfaces and complement the associated conical surface of the other partial piston or the cylinder to form a conical seat together. 

What is claimed is:
 1. A cylinder-piston drive, in particular an hydraulically controlled actuator (1) for actuating a gas-exchange valve (2) of an internal combustion engine, including an operating piston (8) that is displaceable inside a cylinder (6) and delimits pressure chambers (10, 12) by piston sides (14, 16) facing away from one another, the operating piston (8) having a multipart design and being made up of at least two partial pistons (18, 20) that are inserted inside one another and are displaceable relative to each other and strike against one another at stop faces (30, 32, 36, 38), one pressure chamber (10) being delimited by all (18, 20) and the other pressure chamber (12) being delimited by only a part of the partial pistons (20) and the sliding paths (s₁) of the partial pistons (18) not delimiting the other pressure chamber (12) being reduced compared to the overall sliding path (s₁+S₂) of the operating piston (8), and at least one stop face (36), arranged on the cylinder (6), being provided against which a stop face (38) of one of the partial pistons (18) strikes after traveling the reduced sliding path (s₁), wherein at least some of the stop faces associated with each other are designed as conical surfaces (30, 32, 36, 38) that form a conical seat (40, 42) when striking against one another.
 2. The cylinder-piston drive as recited in claim 1, wherein the cone angles of the conical surfaces (30, 32, 36, 38) associated with one another have a slight angle difference and contact one another essentially in the form of a line contact (44, 46).
 3. The cylinder-piston drive as recited in claim 2, wherein the partial pistons (18, 20) have different axial lengths.
 4. The cylinder-piston drive as recited in claim 3, wherein the operating piston (8) is made up of two partial pistons, an outer cylindrical piston (18) having the reduced sliding path (s₁) having a smaller axial length than an inner stepped piston (20) traveling the entire sliding path (s₁+S₂).
 5. The cylinder-piston drive as recited in claim 4, wherein the inner stepped piston (20) is joined to a piston rod (4) or is integrally formed therewith.
 6. The cylinder-piston drive as recited in claim 4 or 5, wherein the cylinder (6) has a bored step (22), a cylinder section (24), having a larger diameter, accommodating both partial pistons (18, 20), and another cylinder section (26), having a smaller diameter, guiding only the stepped piston (20).
 7. The cylinder-piston drive as recited in claim 6, wherein the end of the stepped piston (20) facing the one pressure chamber (10) has a radially outer conical surface (30) that cooperates with an associated radially inner conical surface (32) of the cylindrical piston (18) formed at an annular projection (34).
 8. The cylinder-piston drive as recited in claim 6 or 7, wherein the sliding path of the outer cylindrical piston (18) is able to be limited by a radially inner conical surface (36) formed at the bore step (22) of the cylinder (6), the outer cylindrical piston (18) being provided with an associated radially outer conical surface (38) at its end facing the other pressure chamber (12).
 9. The cylinder-piston drive as recited in claim 7 or 8, wherein, when struck, the radially inner conical surface (32) of the cylindrical piston (18) and the conical surface (30) of the stepped piston (20) and/or the radially outer conical surface (38) of the cylindrical piston (18) and the conical surface (36) of the cylinder (6), form a conical seat (40, 42) in each case. 